The present invention relates to personal radiation dosemeters or indicators which operate to measure radioactivity doses to which personnel are exposed.
Electronic personal dosemeters are worn by personnel in potentially hazardous environments. An example of such an environment is a nuclear power station where there is the possibility, however small, of personnel receiving radiation doses which may be potentially hazardous to their health. For this reason, personnel are often required to carry or wear an electronic personal dosemeter which operates to detect radiation and provide an indication of an amount of radiation to which the personnel are exposed at any time.
Electronic personal dosemeters are provided with radiation detectors which operate to generate a signal for each particle of radiation detected. Electronic personal dosemeters may furthermore provided with a means for converting signals generated or detected by the radiation detector into an audio signal so that a person wearing the dosemeter is given an audible indication of a current level of radiation to which their body is being exposed.
Typically, during a day""s activity, a person working in a power station may be in an environment where there are relatively large amounts of radiation during one part of the day, and in an environment in which there is virtually no radiation, or only background radiation present, in another part of the day. As such, a technical problem exists in that, if the electronic personal detector worn by a person should malfunction at a time when the person is in an environment where there is no radiation, then the person wearing the detector will not be alerted to the presence of harmful amounts of radiation as a result of the malfunction of the dosemeter. A technical problem therefore exists in arranging for the radiation dosemeter to be tested in a way which provides regular confirmation of the integrity of the radiation dose-meter.
The technical problem of testing and confirming the integrity of a radiation dosemeter is addressed by the present invention.
According to the present invention, there is provided a personal radiation dosemeter comprising a radiation detector means coupled to an electronic measurement circuit arranged in combination therewith to generate signals representative of an amount of radiation detected by said radiation detector, wherein said radiation detector means is provided with a light source optically coupled to said radiation detector and arranged to operate under control of a test control circuit to generate light of a wavelength which may be detected by said radiation detector, thereby providing in combination with said electronic measurement circuit an integrity test for said radiation detector means.
The term light as used herein refers to light in both the visible and invisible spectra.
By arranging for the electronic personal dosemeter to be provided with a light source capable of generating photons in the visible or infra-red spectra, and arranging for the test control circuit to generate or energise the light source at predetermined intervals, the electronic personal dosemeter is provided with a means for testing the integrity of the radiation detector and the electronic circuit means thereby confirming the integrity of the radiation dosemeter.
The light source may be a light emitting diode. The optical coupling may include a fibre optic. The optical coupling may be effected from reflection from a surface of the shield.
A further problem with known electronic personal dosemeters is that he radiation detectors are susceptible of providing false readings as a result of electromagnetic interference. For example, stray electromagnetic fields generated by computer monitors, radar systems or mobile phones may cause a false reading of a radiation particle to be detected.
The radiation detector should also be provided with a screen or shield from certain low energy X-rays or gamma photons in order to filter such low energy X-rays or gamma photons in order to provide a better representation of radiation received by the human body. To this end, the radiation detector must be provided with a radiological shield as well as a shield from electromagnetic interference. A technical problem therefore exists in providing a personal radiation dosemeter with a shield from both electromagnetic interference and a radiological shield. This technical problem is addressed by a first aspect of the present invention.
According to a first aspect of the invention there is provided a personal radiation dosemeter comprising a radiation detector means coupled to an electronic measurement circuit and arranged in combination to generate signals representative of an amount of radiation received by said radiation detector means, wherein there is further provided a shield arranged to be electrically coupled to an earth plane and to substantially surround a volume in which said radiation detector means is disposed, said shield being fabricated from electrically conductive material so as to provide substantial electromagnetic screening and said electrically conductive material has a composition and density which is sufficiently high to provide substantial radiological shielding for substantially low energy radiation particles, said shield being thereby arranged to provide both electromagnetic and radiological screening.
The electrically conductive material may be a metal. The metal may be tin. The metal may be an alloy such as pewter.
By fabricating a shield which provides both radiological and electromagnetic screening to the radiation detector, a reduction in size and weight of the radiation dosemeter may be effected. This is particularly advantageous for personal dosemeters.
Known radiation dosemeters are arranged to provide an audible signal for an integral number of particles of radiation detected by a radiation detector. However, there is a requirement to provide instruments to detect radiation, with different characteristics. This requires the use of more than one detector, each of which is arranged to detect radiation with different characteristics. A technical problem therefore exists in providing an audible signal which is representative of the radiation dose rate received by the human body without dependence upon the characteristics of the radiation detector.
Known radiation dose rate meters use a microprocessor to effect processing and combination of signals from a plurality of detectors. However, microprocessors consume a significant amount of power whilst in an operating mode and for this reason may be only activated on a basis of a duty cycle. The duty cycle has an effect that battery power is economised. However the duty cycle of the microprocessor is such that a maximum repetition frequency of the audio signal indicative of the radiation dose rate may be unacceptably low. These technical problems are addressed by a second aspect of the present invention.
According to a second aspect of the present invention there is provided a personal radiation dosemeter for generating a monitor signal representative of a radiation dose rate, said radiation dosemeter comprising a radiation detector means coupled to an electronic measurement circuit arranged in combination therewith to generate signals representative of an amount of radiation received by said radiation detector, wherein said electronic measurement circuit includes at least one data store, an accumulator means and control circuit means, which control circuit means is coupled to said radiation detector means and arranged to add a predetermined number stored in said data store to an accumulated total stored in said accumulator in response to signals from said radiation detector means, said control circuit being arranged to generate a monitor signal for each increment of said accumulated total by said predetermined numerical threshold, which monitor signal is fed to an audio signal generator so as to provide an audible signal in accordance with said increment, a repetition frequency of said audible signal being thereby representative of said radiation dose.
By arranging for a control circuit to add a predetermined number to an accumulator means when a signal is received from the radiation detector means, a scaling of the signal received or generated by the radiation detector means may be effected by appropriately selecting the number stored in the data store and the numerical threshold which is used to trigger the monitor signal when the numerical threshold is reached. In this way scaling of the signal generated by the radiation detector means is effected without a requirement for a microprocessor, thereby allowing the control circuit to be implemented in hardware thus effecting a substantial improvement in power consumption of the electronic personal dosemeter.
The personal dosemeter may further include at least one other radiation detector means and at least one other data store wherein said at least one other data store includes a further predetermined number and said control circuit operates to add said further predetermined number to said accumulator on consequent upon receipt of signals from said at least one other radiation detector means.
By providing a further radiation detector which may be arranged to detect a different type or energy of radiation particle and providing a further data store which is arranged to be pre-stored with a further predetermined number, the control circuit may be arranged to add said further predetermined number when said further radiation detector means detects the presence of radiation. In this way the accumulator maintains a total representative of an amount of radiation detected by said radiation detector means and said further radiation detector means in proportion to the first and further predetermined numbers. By selecting the first and the further predetermined numbers independence upon the numerical threshold, the radiation dosemeter may be arranged to generate a monitor signal at a repetition frequency which is arranged to provide an audible indication determined by the relative amounts and harmful effect of the radiation detected by the first and further radiation detector means.